Gas turbine engine with intercooled cooling air and dual towershaft accessory gearbox

ABSTRACT

An exemplary gas turbine engine assembly includes a first spool having a first turbine operatively mounted to a first turbine shaft, and a second spool having a second turbine operatively mounted to a second turbine shaft. The first and second turbines are mounted for rotation about a common rotational axis within an engine static structure. The first and second turbine shafts are coaxial with one another. First and second towershafts are respectively coupled to the first and second turbine shafts. An accessory drive gearbox has a set of gears. A compressor is driven by the first towershaft. The engine assembly further includes a starter generator assembly, and a transmission coupling the starter generator assembly to the first set of gears. The transmission is transitionable between a first mode where the starter generator assembly is driven at a first speed relative to the second towershaft, and a second mode where the starter generator assembly is driven at a different, second speed relative to the second towershaft.

BACKGROUND

A gas turbine engine typically includes a fan section, a compressorsection, a combustor section, and a turbine section. Air entering thecompressor section is compressed and delivered into the combustorsection where it is mixed with fuel and ignited to generate a high-speedexhaust gas flow. The high-speed exhaust gas flow expands through theturbine section to drive the compressor and the fan section. Thecompressor section typically includes low and high pressure compressors,and the turbine section includes low and high pressure turbines.

As known, the turbine components see very high temperatures. As such, itis known to deliver cooling air to the turbine. In particular, reducinga size of an engine core can increases the compressor exit pressures andtemperatures and, at the same time, increase the turbine temperatures

The higher temperatures and pressures at the upstream end of the turbinesection raise cooling challenges. This is where one branch of thecooling air is typically delivered. As such, the cooling air must be ata sufficiently high pressure that it can move into this environment.

Historically, air from near a downstream end of the compressor sectionhas been tapped to provide cooling air. However, with a greater emphasison engine fuel burn, the efficient use of all air delivered into thecore engine becomes more important. As such, utilizing air which hasalready been fully compressed is undesirable.

Recently, it has been proposed to tap the cooling air from a locationupstream of the downstream most location in the compressor. This air isthen passed through a boost compressor, which increases its pressuresuch that it now can move into the turbine section.

A typical gas turbine engine utilizes one or more gearboxes to driveaccessory components, such as generators, fuel pumps, and oil pumps.Each of the accessory drive components must be driven at a desiredrotational speed. As a result, the accessory is coupled to either thelow or high speed spool and geared accordingly to obtain the speed atwhich the accessory operates more efficiently.

One gearbox has been proposed in which the accessory drive componentsare driven by a single towershaft. Other gearboxes have been proposed inwhich some accessory drive components are driven by a first towershaft,and other accessory drive components are driven by a second towershaft.

SUMMARY

In one exemplary embodiment a gas turbine engine assembly includes afirst spool having a first turbine operatively mounted to a firstturbine shaft, and a second spool having a second turbine operativelymounted to a second turbine shaft. The first and second turbines aremounted for rotation about a common rotational axis within an enginestatic structure. The first and second turbine shafts are coaxial withone another. First and second towershafts are respectively coupled tothe first and second turbine shafts. An accessory drive gearbox has aset of gears. A compressor is driven by the first towershaft. The engineassembly further includes a starter generator assembly, and atransmission coupling the starter generator assembly to the first set ofgears. The transmission is transitionable between a first mode where thestarter generator assembly is driven at a first speed relative to thesecond towershaft, and a second mode where the starter generatorassembly is driven at a different, second speed relative to the secondtowershaft.

In another example of the above described gas turbine engine assembly,the first and second turbine shafts are outer and inner shafts,respectively, and the first and second turbines are high and lowpressure turbines, respectively.

In another example of any of the above described gas turbine engineassemblies, the first towershaft is configured to rotate at a higherspeed than the second towershaft.

In another example of any of the above described gas turbine engineassemblies, the transmission is further transitionable to a third modewhere the starter generator assembly is driven at a different, thirdspeed relative to the second towershaft.

In another example of any of the above described gas turbine engineassemblies, the transmission is further transitionable to at least onefourth mode where the starter generator assembly is driven at a fourthspeed relative to the second towershaft, the fourth speed different thaneach of the first, second, and third speeds.

Another example of any of the above described gas turbine engineassemblies further includes a first clutch disposed between the firsttowershaft and the starter generator assembly. The first clutch isconfigured to enable the starter generator assembly to drive the firstspool through the accessory drive gearbox, and a second clutch disposedbetween the second towershaft and the starter generator assembly, thesecond clutch configured to enable the second spool to drive the startergenerator assembly through the accessory drive gearbox.

In another example of any of the above described gas turbine engineassemblies, the first clutch and the second clutch are one-waymechanical clutch devices.

In another example of any of the above described gas turbine engineassemblies, the compressor is a boost compressor of an intercooledcooling air system.

Another example of any of the above described gas turbine engineassemblies further includes a compressor section having a downstreammost end and a cooling air tap at a location spaced upstream from saiddownstream most end. The cooling air tap is passed through at least oneboost compressor and at least one heat exchanger, and then passed to aturbine section to cool the turbine section.

Another example of any of the above described gas turbine engineassemblies further includes a fan driven by a speed reduction device.The speed reduction device is driven by the second turbine shaft.

In another example of any of the above described gas turbine engineassemblies, the starter generator assembly comprises a first variablefrequency generator and a second variable frequency generator.

In another example of any of the above described gas turbine engineassemblies, the starter generator assembly comprises a first integrateddrive generator and a second integrated drive generator.

An exemplary method of operating a gas turbine engine includes driving afirst spool with a starter through a first towershaft and a first clutchto start the engine, driving a starter generator assembly through anaccessory gearbox through a second clutch with a second towershaftcoupled to a second spool once the engine is started, and driving acompressor through the accessory gearbox with the first towershaft oncethe engine is started.

Another example of the above described exemplary method of operating agas turbine engine further includes decoupling the starter from thefirst spool once the first spool reaches an engine idle speed.

In another example of any of the above described methods of operating agas turbine engine, the decoupling includes rotating the secondtowershaft at a speed greater than that of the starter.

In another example of any of the above described methods of operating agas turbine engine, the compressor is a boost compressor of anintercooled cooling air system.

Another example of any of the above described methods of operating a gasturbine engine further includes a compressor section having a downstreammost end and a cooling air tap at a location spaced upstream from saiddownstream most end, wherein the cooling air tap is passed through atleast one boost compressor and at least one heat exchanger, and thenpassed to a turbine section to cool the turbine section.

Another example of any of the above described methods of operating a gasturbine engine further includes driving the starter generator through atransmission in a first mode so that the starter generator assembly isrotated at a first speed relative to the second towershaft, andtransitioning the transmission to a second mode so that the startergenerator is driven by the second towershaft and rotated at a different,second speed relative to the second towershaft.

Another example of any of the above described methods of operating a gasturbine engine further comprising transitioning the transmission to athird mode; and driving the transmission with the second towershaft torotate the starter generator assembly at a different, third speedrelative to the second towershaft.

Another example of any of the above described method of operating a gasturbine engine further includes driving a fan through a speed reductiondevice with a shaft of the second spool.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be further understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 schematically illustrates a gas turbine engine embodiment.

FIG. 2 is a schematic view illustrating a common accessory drive gearboxdriven by both high and low speed spools.

FIG. 3 is a schematic view of the example accessory gearbox of FIG. 2and towershafts.

FIG. 4 is a perspective view of the accessory drive gearbox of FIG. 2with the accessory drive components mounted thereto.

FIG. 5 is a schematic view of an intercooled cooling air system with theengine of FIG. 1.

FIG. 6 shows a schematic view of a portion of the accessory drivegearbox of FIG. 4 coupled to a starter generator assembly through atransmission in a first mode.

FIG. 7 shows a schematic view of a portion of the accessory drivegearbox of FIG. 4 coupled to the starter generator assembly through thetransmission in a second mode.

FIG. 8 shows a schematic view of a portion of the accessory drivegearbox of FIG. 4 coupled to the starter generator assembly through thetransmission in a third mode.

FIG. 9 is a schematic view of accessory gearbox operation during astarting process.

FIG. 10 is a schematic view of accessory gearbox operation during engineoperation.

The embodiments, examples and alternatives of the preceding paragraphs,the claims, or the following description and drawings, including any oftheir various aspects or respective individual features, may be takenindependently or in any combination. Features described in connectionwith one embodiment are applicable to all embodiments, unless suchfeatures are incompatible.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. Alternative engines mightinclude an augmentor section (not shown) among other systems orfeatures. The fan section 22 drives air along a bypass flow path B in abypass duct defined within a nacelle 15, while the compressor section 24drives air along a core flow path C for compression and communicationinto the combustor section 26 then expansion through the turbine section28. Although depicted as a two-spool turbofan gas turbine engine in thedisclosed non-limiting embodiment, it should be understood that theconcepts described herein are not limited to use with two-spoolturbofans as the teachings may be applied to other types of turbineengines including three-spool architectures.

The exemplary engine 20 generally includes a low speed spool 30 and ahigh speed spool 32 mounted for rotation about an engine centrallongitudinal axis A relative to an engine static structure 36 viaseveral bearing systems 38. It should be understood that various bearingsystems 38 at various locations may alternatively or additionally beprovided, and the location of bearing systems 38 may be varied asappropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects a fan 42, a first (or low) pressure compressor 44 and afirst (or low) pressure turbine 46. The inner shaft 40 is connected tothe fan 42 through a speed change mechanism, which in exemplary gasturbine engine 20 is illustrated as a geared architecture 48 to drivethe fan 42 at a lower speed than the low speed spool 30. The high speedspool 32 includes an outer shaft 50 that interconnects a second (orhigh) pressure compressor 52 and a second (or high) pressure turbine 54.A combustor 56 is arranged in exemplary gas turbine engine 20 betweenthe high pressure compressor 52 and the high pressure turbine 54. Amid-turbine frame 58 of the engine static structure 36 is arrangedgenerally between the high pressure turbine 54 and the low pressureturbine 46. The mid-turbine frame 58 further supports bearing systems 38in the turbine section 28. The inner shaft 40 and the outer shaft 50 areconcentric and rotate via bearing systems 38 about the engine centrallongitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 58 includes airfoils 60 whichare in the core airflow path C. The turbines 46, 54 rotationally drivethe respective low speed spool 30 and high speed spool 32 in response tothe expansion. It will be appreciated that each of the positions of thefan section 22, compressor section 24, combustor section 26, turbinesection 28, and geared architecture 48 may be varied. For example,geared architecture 48 may be located aft of combustor section 26 oreven aft of turbine section 28, and fan section 22 may be positionedforward or aft of the location of gear architecture 48.

The engine 20 in one example is a high-bypass geared aircraft engine. Ina further example, the engine 20 bypass ratio is greater than about six(6), with an example embodiment being greater than about ten (10), thegeared architecture 48 is an epicyclic gear train, such as a planetarygear system or other gear system, with a gear reduction ratio of greaterthan about 2.3 and the low pressure turbine 46 has a pressure ratio thatis greater than about five. In one disclosed embodiment, the engine 20bypass ratio is greater than about ten (10:1), the fan diameter issignificantly larger than that of the low pressure compressor 44, andthe low pressure turbine 46 has a pressure ratio that is greater thanabout five 5:1. Low pressure turbine 46 pressure ratio is pressuremeasured prior to inlet of low pressure turbine 46 as related to thepressure at the outlet of the low pressure turbine 46 prior to anexhaust nozzle. The geared architecture 48 may be an epicycle geartrain, such as a planetary gear system or other gear system, with a gearreduction ratio of greater than about 2.3:1. It should be understood,however, that the above parameters are only exemplary of one embodimentof a geared architecture engine and that the present invention isapplicable to other gas turbine engines including direct driveturbofans.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the engine 20 is designedfor a particular flight condition—typically cruise at about 0.8 Mach andabout 35,000 feet (10.67 km). The flight condition of 0.8 Mach and35,000 ft (10.67 km), with the engine at its best fuel consumption—alsoknown as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—isthe industry standard parameter of lbm of fuel being burned divided bylbf of thrust the engine produces at that minimum point. “Low fanpressure ratio” is the pressure ratio across the fan blade alone,without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressureratio as disclosed herein according to one non-limiting embodiment isless than about 1.45. “Low corrected fan tip speed” is the actual fantip speed in ft/sec divided by an industry standard temperaturecorrection of [(Tram ° R)/(518.7° R)]^(0.5). The “Low corrected fan tipspeed” as disclosed herein according to one non-limiting embodiment isless than about 1150 ft/second (350 m/second).

The example gas turbine engine includes the fan 42 that comprises in onenon-limiting embodiment less than about twenty-six (26) fan blades. Inanother non-limiting embodiment, the fan section 22 includes less thanabout twenty (20) fan blades. Moreover, in one disclosed embodiment thelow pressure turbine 46 includes no more than about six (6) turbinerotors schematically indicated at 34. In another non-limiting exampleembodiment the low pressure turbine 46 includes about three (3) turbinerotors. A ratio between the number of fan blades and the number of lowpressure turbine rotors is between about 3.3 and about 8.6. The examplelow pressure turbine 46 provides the driving power to rotate the fansection 22 and therefore the relationship between the number of turbinerotors 34 in the low pressure turbine 46 and the number of blades in thefan section 22 disclose an example gas turbine engine 20 with increasedpower transfer efficiency.

The example engine 20 includes a first towershaft 64 that is engaged todrive the high speed spool 32. The engine 20 further includes a secondtowershaft 66 that is engaged to be driven by the low speed spool 30.The low speed spool 30 includes a gear 70 and the high speed spool 32includes a gear 68. The gear 68 is engaged to the first towershaft 64and the gear 70 is engaged to the second towershaft 66. In one disclosedembodiment the gears 68 and 70 are bevel gears and engage correspondingbevel gears on the corresponding first or second towershaft 64, 66.

Referring to FIG. 2 with continued reference to FIG. 1, the examplegearbox 62 includes a gear engagement with both the first towershaft 64and the second towershaft 66. The towershafts 64, 66 interface with acommon accessory gearbox 62 and enable the use of the low speed spool 30to drive the accessory components within the accessory gearbox 62.

Referring to FIG. 3 with continued reference to FIG. 2, a first clutch72 is engaged to the first towershaft 64 coupled to the high speed spool32. A second clutch 74 is disposed on the second towershaft 66 driven bythe low speed spool 30. Each of the clutches 74 and 72 provide for thetransmission of torque in a single direction. The accessory gearbox 62is engaged to a starter generator assembly 76 that drives a set of gears78 meshing with both the first towershaft 64 and the second towershaft66.

In the disclosed example, the clutches 72 and 74 are sprag clutches thatonly allow torque to be transmitted in one direction. When torque isreversed, meaning that the driving member becomes the driven member, theclutch will slip and allow the driving member to overspeed relative tothe driven member. In this example, the second clutch 74 will allow thelow rotor to drive the starter generator assembly 76, but does not allowthe starter generator assembly 76 to drive low speed spool 30. In theFIG. 3 example, the first clutch 72 can be located within the accessorygearbox 62, however, the first clutch 74 may be located whereverpractical to provide the selective application of torque between thestarter generator assembly 76 and the first towershaft 64. Similarly,the second clutch 74 can be located wherever practical to provideselective application of torque.

The first clutch 72 is configured to allow the starter generatorassembly 76 to drive the high speed spool 32 but not to allow the highspeed spool 32 to drive the starter generator assembly 76. In thisexample, because the high speed spool 32 will rotate much faster thanthe starter generator assembly 76, the first clutch 72 is configuredsuch that the high speed spool 32 may over speed past the speed of thestarter generator assembly 76 and not transmit torque to the startergenerator assembly 76 through the first towershaft 64.

Referring now to FIG. 4 with continuing reference to FIG. 3, theaccessory gearbox 62 is used to drive the starter generator assembly 76,and a number of other accessory components, including, but not limitedto a compressor 80, an oil pump 84, a hydraulic pump 88, and a permanentmagnet alternator (PMA) 90. The first clutch 72 is positioned such thatthe first towershaft 64 can selectively drive the compressor 80 and thepermanent magnet alternator 90. The second clutch 74 is positioned suchthat the second towershaft 66 can selectively drive the oil pump 84, andfurther drive the hydraulic pump 88 and the starter generator assembly76 through a transmission 92. A fuel pump (not shown) can beelectrically driven and powered by the starter generator assembly 76.

After the high speed spool 32 has reached sufficient rotational speed,the first clutch 72 effectively decouples rotation of the firsttowershaft 64, the compressor 80, and the permanent magnet alternator 90from the rotations of the second towershaft 66, the transmission 92, thestarter generator assembly 76, the hydraulic pump 88, and the oil pump84. This permits the first towershaft 64 to drive the compressor 80 andthe permanent magnet alternator 90 while the second towershaft 66 drivesthe remaining accessories.

The compressor 80, in this example, is a boost compressor pump usedwithin an intercooled cooling air (I-CCA) system 100 as shown in FIG. 5.The system 100, generally, permits cooling the high turbine withsecondary air. In the system 100, the fan 42 delivers air into a bypassduct 106 as propulsion air. The fan 42 also delivers air to the lowpressure compressor 44. The air then passes into the high pressurecompressor 52. A tap 112 is shown in the high pressure compressor 52adjacent a downstream most end 113 of the compressor 52. Another tap 114is shown at a location upstream of the downstream most end 113. Aircompressed by the compressor 52 passes into the combustor 56. The air ismixed with fuel and ignited and products of this combustion pass overthe high pressure turbine 54. In this embodiment, there will typicallybe at least a second turbine. In some embodiments, there may be a thirdturbine which drives the fan 42. The geared architecture 48 is shownbetween a shaft 121 driven by a fan drive turbine (which may be thesecond turbine or the third turbine, if one is included).

Air from the tap 114 is utilized as cooling air. It passes through avalve 120 to a heat exchanger 122. The air in the heat exchanger 122 iscooled by the bypass air in duct 106. Of course, other locations for theheat exchanger may be selected. Downstream of the heat exchanger 122 airpasses through the compressor 80. The boost compressor 80 is driven bythe first towershaft 64 through the set of gears 78 in the accessorygearbox 62.

Air downstream of the boost compressor 80 passes through a heatexchanger 128, and then to a mixing chamber 130. It should be understoodthat while two heat exchangers 122 and 128 are illustrated, only oneheat exchanger may be needed. In the mixing chamber 130, air from thedownstream location 112 is mixed with the air from the location 114 toarrive at a desired mix of temperature and pressure to be delivered atline 132 to cool the high pressure turbine 54.

As an example, at lower power operation, more air from the downstreammost location 112 may be utilized with limited disadvantage toefficiency. The mixing chamber 130 may be a passive orifice feature. Aslong as the pressure downstream of the boost compressor 80 is higherthan the air from location 112, the air from the boost compressor 80will flow for cooling. Air from the tap 112 can make up any differencein the required flow volume. A control 134 controls the mixing chamber130. It should be understood that the other valves and other items couldalso be controlled by the control 134. Control 134 may be a standalonecontrol or may be part of a full authority digital electronic controller(FADEC).

Referring again to FIG. 4, the second towershaft 66 drives the hydraulicpump 88 and the starter generator assembly 76 through a transmission 92.The oil pump 84 is driven by the second towershaft 66, but not throughthe transmission 92. The rotational speed of the oil pump 84 varieslinearly with the rotational speed of the second towershaft 66.

In an exemplary non-limiting embodiment, the transmission 92 is athree-speed transmission that can transition between a first mode ofshown schematically in FIG. 6, a second mode shown schematically in FIG.7, and a third mode shown schematically in FIG. 8.

In the first mode, the transmission 92 is rotated by the secondtowershaft 66 and, in response, rotates the hydraulic pump 88 and thestarter generator assembly 76 at a first ratio relative to a rotationalspeed of the second towershaft 66. In the second mode, the transmission92 is rotated by the second towershaft 66 and, in response, rotates thehydraulic pump 88 and the starter generator assembly 76 at a different,second ratio relative to a rotational speed of the second towershaft 66.In the third mode, the transmission 92 is rotated by the secondtowershaft 66 and, in response, rotates the hydraulic pump 88 and thestarter generator assembly 76 at a different, third ratio relative to arotational speed of the second towershaft 66.

During operation, the inner shaft 40 can experience a greater range ofrotational speeds that the outer shaft 50. That is, the speed excursionfor the inner shaft 40 can be higher than the speed excursion for theouter shaft 50. In a specific non-limiting embodiment, the inner shaft40 can operate at speed excursions of up to 80% during operation of thegas turbine engine 20, whereas the outer shaft 50 can operate at speedexcursions of up to 30% during operation of the gas turbine engine 20.

The transmission 92 addresses issues associated with rotating thestarter generator assembly 76 and the hydraulic pump 88 with a rotatableinput from the second towershaft 66. In the exemplary embodiments, thetransmission 92 operates in the first mode when the inner shaft 40 isrotating at a speed excursion of say less than 25%. If the speedexcursion of the inner shaft 40 meets or exceeds 25%, but is less than50%, the transmission 92 switches to the second mode to rotate thestarter generator assembly 76 and the hydraulic pump 88. If the speedexcursion of the inner shaft 40 meets or exceeds 50%, the transmission92 switches to the third mode to rotate the starter generator assembly76 and the hydraulic pump 88.

This permits the second towershaft 66 to drive the starter generatorassembly 76 and the hydraulic pump 88 through the transmission 92 atthree different ratios. The starter generator assembly 76 is thus notrequired to operate across a range of speed excursions from 0 up to 80%during operation of the gas turbine engine 20. Instead, due to thetransmission 92, the range is no more than, say, 30% for the startergenerator assembly 76 and the hydraulic pump 88. The starter generatorassembly 76 can operate more efficiently when the starter generatorassembly 76 and the hydraulic pump 88 is rotated across a smaller rangeof rotational speeds than across a larger range of rotational speeds.

An electronic engine control (EEC) 96 can control the transition of thetransmission 92 between the first mode, the second mode, and the thirdmode. The EEC 96 could, for example, receive an input corresponding tothe rotational speed of the inner shaft 40, and then transition thetransmission 92 from the first mode to the second mode or the third modewhen the rotational speed exceeds a threshold speed.

Although the exemplary transmission 92 can transition between threemodes, other exemplary embodiments of the transmission 92 couldtransition between more than three modes. In such examples, thetransmission 92 is rotated by the second towershaft 66 through the firstset of gears 78 and, in response, rotates the starter generator assembly76 and the hydraulic pump 88 at four or more different ratios relativeto a rotational speed of the second towershaft 66.

Referring to FIG. 9, the example accessory gearbox 62 is shown during anengine starting operation. In this schematic illustration, the startergenerator assembly 76 is shown driving the set of gears 78 within theaccessory gearbox 62 that, in turn, drives the first clutch 72 andthereby the first towershaft 64 to drive the high speed spool 32 up to aspeed required for starting of the engine 20. The same set of gears 78driven by the starter generator assembly 76 is also driving the secondclutch 74 that is engaged to the second towershaft 66 driven by the lowspeed spool 30. However, in this instance, the second clutch 74 is nottransmitting torque to the low speed spool 30. Accordingly, in theconfiguration schematically illustrated in FIG. 9, only the high speedspool 32 is turning.

Once the high speed spool 32 has been spun up to operating conditions,it will attain a speed that is much greater than that input by thestarter generator assembly 76 and the first towershaft 64. The firsttowershaft 64 will continue to rotate in a direction originally providedby the starter generator assembly 76, however, the high speed spooldriven towershaft 64 is rotating at a much higher speed and thereforespin past the speed input by the starter generator assembly 76. Thefirst clutch 72 will not allow the transmission of this higher torquefrom the high speed spool 32 to accessory components other than thepermanent magnet alternator 90 and the compressor 80.

Once the high speed spool 32 has become operational, the low speed spool30 will also begin to turn and shown in FIG. 10. Rotation of the highspeed spool will result in turning of the second towershaft 66. Thesecond towershaft 66 will in turn, turn the set of gears 78 through thesecond clutch 74 which will drive the starter generator assembly 76.Because the second clutch 74 is orientated and configured to enable thelow speed spool 30 to drive the towershaft 66 and in turn drive thestarter generator assembly 76.

Accordingly, once the engine is running, the starter generator assembly76 may produce electric power to drive any number of accessory units.Moreover, once the engine is operational, the accessory components canbe electrically powered by the starter generator assembly 76 ormechanically powered by the low speed spool 30.

In this example, the starter generator assembly 76 comprises twoseparate variable frequency generators. The variable frequencygenerators are each rated at 90 kVA in some examples. Notably, becausesince the starter generator assembly 76 can start the engine 20, noseparate starter is required. When starting the engine 20, both starterscan be powered to shorten start time. Prior to an engine startprocedure, one of the starter-generators can be engaged to turn the highspeed spool and reduced a bowed rotor start.

The variable frequency generators receives a rotational input togenerate power utilized by components of the gas turbine engine 20.Other examples could incorporate other types of generators, and othertypes of electric machines. The transmission 92 facilitatesincorporating the variable frequency generators rather than, forexample, an integrated drive generator, since the transmission 92permits operating the starter generator assembly 76 to operate in anarrower rpm range while still being driven by rotation of the innershaft 40 through the second towershaft 66. Other examples, however,could include using one or more integrated drive generators as thestarter generator assembly 76.

Driving the starter generator assembly 76 with the inner shaft 40 whengenerating power, rather than the outer shaft 50, can improve engineoperability and performance. The exhaust gas temperature is also reducedas there is less power draw on the outer shaft 50.

The hydraulic pump 88 (FIG. 4) generally moves hydraulic fluid needed tomove components of an air frame to which the gas turbine engine 20 ismounted. The transmission 92 permits operating the hydraulic pump 88 tobe driven in a narrower rpm range while still being driven by rotationof the inner shaft 40 through the second towershaft 66. Driving thehydraulic pump 88 with the inner shaft 40, rather than the outer shaft50, can improve engine operability and performance. The exhaust gastemperature is also reduced as there is less power draw on the outershaft 50.

The oil pump 84 is driven at a fixed ratio relative to the speed of thesecond towershaft 66. That is, switching the transmission 92 between thevarious modes does not substantially change a ratio of rotational speedsbetween the second towershaft 66 and the oil pump 84. Thus, as therotational speed of the second towershaft 66 varies, the rotationalinput to the oil pump 84 varies linearly with the rotational speed ofthe second towershaft 66.

The oil pump 84 can be dedicated to supplying oil for all low rotorbearings whether roller, ball, or tapered bearings, and furtherincluding supplying oil for the geared architecture 48 or fan drive gearsystem. In another example, the oil pump 84 is dedicated to supplyingoil to the low rotor bearing system 38′, which incorporates bearingsdirectly supporting the inner shaft 40. The bearings of the low rotorbearing system 38′ are tapered bearings in some examples.

The rotational speed of the second towershaft 66 increases when therotational speed of the inner shaft 40 increases. The inner shaft 40 mayrequire additional lubrication, such as oil, directed to bearing systems38 supporting the inner shaft 40 when the rotational speed of the innershaft 40 increases.

The increased lubrication demands due to increasing the rotational speedof the inner shaft 40 are met by increasing the rotational input speedto the oil pump 84. In other words, the amount of oil moved to thebearing system 38′ varies linearly with the rotational speed of theinner shaft 40. If the oil pump 84 were instead varying linearly withthe rotational speed of the outer shaft 50, the oil pump 84 may movemore oil than is required for lubrication. The excess oil would need torecirculated, or accommodated in some other way, which results inlosses.

The oil pump 84 is considered a 60% oil pump as it accommodatesapproximately 60% of the lubrication requirements for the gas turbineengine 20. An additional pump, not shown, such as an electric pump,could be incorporated into the engine and powered by the startergenerator assembly 76 to supply lubricant to other areas of the gasturbine engine 20.

The permanent magnet alternator 90 can be used to power the FADEC, whichcan include the EEC 96. As the FADEC is used during start up, thepermanent magnet alternator 90 is also driven by the outer shaft 50.

Referring again to the oil pump 84, an added feature of couplingrotation of the oil pump 84 with rotation of the inner shaft 40 is thatthe inner shaft 40 spins with the fan 42. Thus, during a windmillingevent when the fan 42 is spinning without being driven by the innershaft 40, the oil pump 84 can continue to pump oil lubricating thebearings associated with the inner shaft 40. If the oil pump 84 weredecoupled from rotation with the inner shaft 40, another pump or anelectronic pump could be required to move oil to lubricate the fan 42when windmilling.

Accordingly, some of the exemplary accessory gearboxes and relatedcomponents enable the use of compact high speed spool systems to enhanceefficiencies.

Some of the disclosed exemplary embodiments can be used to replace fueland oil pumps with electrically powered on-demand fuel and oil pumps. Insome examples, the fuel pump formerly driven by the accessory gearbox isreplaced with a compressor that is part of an intercooled cooling airsystem.

Some of the exemplary embodiments can improve fuel burn overarrangements with a starter generator assembly driven in a generatormode by the high spool. Since the starter generators drive the highspool when the engine is started, and then transitioned away from thehigh spool, no additional starter on the high spool is required. Usingthe starter-generators can reduce idle thrust and ground/flight idleexhaust gas temperatures. The starter-generators can facilitate winganti-ice.

It should also be understood that although a particular componentarrangement is disclosed in the illustrated embodiment, otherarrangements will benefit herefrom. Although particular step sequencesare shown, described, and claimed, it should be understood that stepsmay be performed in any order, separated or combined unless otherwiseindicated and will still benefit from the present invention.

Although the different examples have specific components shown in theillustrations, embodiments of this invention are not limited to thoseparticular combinations. It is possible to use some of the components orfeatures from one of the examples in combination with features orcomponents from another one of the examples.

Although an example embodiment has been disclosed, a worker of ordinaryskill in this art would recognize that certain modifications would comewithin the scope of the claims. For that reason, the following claimsshould be studied to determine their true scope and content.

What is claimed is:
 1. A gas turbine engine assembly comprising: a firstspool having a first turbine operatively mounted to a first turbineshaft; a second spool having a second turbine operatively mounted to asecond turbine shaft, the first and second turbines mounted for rotationabout a common rotational axis within an engine static structure, thefirst and second turbine shafts coaxial with one another; first andsecond towershafts respectively coupled to the first and second turbineshafts; an accessory drive gearbox with a set of gears; a compressordriven by the first towershaft; a starter generator assembly; and atransmission coupling the starter generator assembly to the set ofgears, the transmission transitionable between a first mode where thestarter generator assembly is driven at a first speed relative to thesecond towershaft, and a second mode where the starter generatorassembly is driven at a different, second speed relative to the secondtowershaft; and wherein the transmission is further transitionable to athird mode where the starter generator assembly is driven at adifferent, third speed relative to the second towershaft.
 2. The gasturbine engine assembly of claim 1, wherein the first and second turbineshafts are outer and inner shafts, respectively, and the first andsecond turbines are high and low pressure turbines, respectively.
 3. Thegas turbine engine assembly of claim 2, wherein the first towershaft isconfigured to rotate at a higher speed than the second towershaft. 4.The gas turbine engine assembly of claim 1, wherein the transmission isfurther transitionable to at least one fourth mode where the startergenerator assembly is driven at a fourth speed relative to the secondtowershaft, the fourth speed different than each of the first, second,and third speeds.
 5. The gas turbine engine assembly of claim 1, furthercomprising: a first clutch disposed between the first towershaft and thestarter generator assembly, the first clutch configured to enable thestarter generator assembly to drive the first spool through theaccessory drive gearbox; and a second clutch disposed between the secondtowershaft and the starter generator assembly, the second clutchconfigured to enable the second spool to drive the starter generatorassembly through the accessory drive gearbox.
 6. The gas turbine engineassembly of claim 5, wherein the first clutch and the second clutch areone-way mechanical clutch devices.
 7. The gas turbine engine assembly ofclaim 5, wherein the compressor is a boost compressor of an intercooledcooling air system, and further comprising: a fan section including afan driven by the second turbine through a geared architecture, whereinthe fan section drives air long a bypass flow path in a bypass duct; acompressor section including a low pressure compressor and a highpressure compressor, the high pressure compressor driven by the firstturbine, and the low pressure compressor driven by the second turbinethrough an epicyclic gear train; wherein the fan section drives air longthe bypass flow path in the bypass duct, a bypass ratio is defined asthe volume of air passing into the bypass duct compared to the volume ofair passing into the compressor section, and the bypass ratio is greaterthan 10; wherein the first and second turbine shafts are outer and innershafts, respectively, and the first and second turbines are high and lowpressure turbines, respectively; and wherein the first towershaft isconfigured to rotate at a higher speed than the second towershaft. 8.The gas turbine engine assembly of claim 7, wherein the compressorsection has a downstream most end and a cooling air tap at a locationspaced upstream from the downstream most end, wherein the cooling airtap is passed through the boost compressor and at least one heatexchanger, and then passed to a turbine section to cool the turbinesection, and the turbine section comprises the high and low pressureturbines.
 9. The gas turbine engine assembly of claim 1, wherein thecompressor is a boost compressor of an intercooled cooling air system.10. The gas turbine engine assembly of claim 9, further comprising acompressor section having a downstream most end and a cooling air tap ata location spaced upstream from said downstream most end, wherein thecooling air tap is passed through at least one boost compressor and atleast one heat exchanger, and then passed to a turbine section to coolthe turbine section.
 11. The gas turbine engine assembly of claim 1,further comprising a fan driven by a speed reduction device, wherein thespeed reduction device is driven by the second turbine shaft.
 12. Thegas turbine engine assembly of claim 1, wherein the starter generatorassembly comprises a first variable frequency generator and a secondvariable frequency generator.
 13. The gas turbine engine assembly ofclaim 1, wherein the starter generator assembly comprises a firstintegrated drive generator and a second integrated drive generator. 14.A method of operating a gas turbine engine, comprising: driving a firstspool with a starter generator assembly through a first towershaft and afirst clutch to start the engine; driving the starter generator assemblythrough an accessory gearbox through a second clutch with a secondtowershaft coupled to a second spool once the engine is started; anddriving a compressor through the accessory gearbox with the firsttowershaft once the engine is started.
 15. The method of claim 14,further comprising decoupling the starter generator assembly from thefirst spool once the first spool reaches an engine idle speed.
 16. Themethod of claim 15, wherein the decoupling includes rotating the secondtowershaft at a speed greater than that of the starter generatorassembly.
 17. The method of claim 14, wherein the compressor is a boostcompressor of an intercooled cooling air system.
 18. The method of claim17, further comprising a compressor section having a downstream most endand a cooling air tap at a location spaced upstream from said downstreammost end, wherein the cooling air tap is passed through the boostcompressor and at least one heat exchanger, and then passed to a turbinesection to cool the turbine section.
 19. The method of claim 18, whereinthe compressor section includes a low pressure compressor and a highpressure compressor, the low pressure compressor driven by a lowpressure turbine and the high pressure compressor driven by a highpressure turbine, the first spool comprising the high pressurecompressor and the high pressure compressor, and the second spoolcomprising the low pressure compressor and the low pressure turbine, andfurther comprising: driving the starter generator assembly through theaccessory gearbox through the second clutch with the second towershaftsubsequent to the step of decoupling the starter generator assembly fromthe first spool; and driving the boost compressor through the accessorygearbox with the first towershaft subsequent to the step of decouplingthe starter generator assembly from the first spool.
 20. The method ofclaim 14, further comprising: driving the starter generator assemblythrough a transmission in a first mode so that the starter generatorassembly is rotated at a first speed relative to the second towershaft;and transitioning the transmission to a second mode so that the startergenerator assembly is driven by the second towershaft and rotated at adifferent, second speed relative to the second towershaft.
 21. Themethod of claim 20, further comprising transitioning the transmission toa third mode; and driving the transmission with the second towershaft torotate the starter generator assembly at a different, third speedrelative to the second towershaft.
 22. The method of claim 14, furthercomprising driving a fan through a speed reduction device with a shaftof the second spool.